1. Field of the Invention
This invention relates to a solventless or high-solids epoxy resin coating composition mainly for use in forming anticorrosive coatings which comprises an epoxy resin containing in the molecule at least 2 epoxy groups and a curing agent composition.
2. Description of the Prior Art
Vehicles now used for paints include a variety of resins, for example, unsaturated fatty acids, alkyd resins, olefin type resins, diene type resins, acrylic resins, polyester resins, epoxy resins, urethane resins and copolymers of these resins. Among them, epoxy resins are excellent in physical properties (adhesion to substrates and hardness) and chemical properties (resistances to chemicals) and the demand for epoxy resins as vehicles for paints has been increasing.
Recently, it has become an important requirement that paints should not be dangerous or harmful. Namely, paints are required to have a much reduced inflammability and no toxicity to the human body.
As one paint meeting such requirements, there has been developed a so-called aqueous paint comprising an epoxy resin dispersed, emulsified or dissolved in water. Coating epoxy resin compositions of this type are disclosed in, for example, the specifications of U.S. Pat. No. 2,811,495, U.S. Pat. No. 2,899,397, U.S. Pat. No. 3,324,041, U.S. Pat. No. 3,355,409, U.S. Pat. No. 3,449,281 and U.S. Pat. No. 3,640,926.
A variety of other resin compositions have also been investigated and developed in the art as aqueous paints.
However, these known aqueous coating resin compositions are still inferior to solvent type coating compositions with respect to such properties as moisture resistance, corrosion resistance and physical properties.
Various attempts have heretofore been made to reduce or overcome these shortcomings. For example, addition of rust inhibiting pigments such as lead cyanamide, lead suboxide, basic lead chromate, red lead, strontium chromate and zinc chromate has been proposed, but use of these pigments is not desirable because these pigments readily cause environmental pollution.
Further, none of the known aqueous coating resin compositions are completely satisfactory in various physical properties represented by the adhesion under a high humidity condition.
Further, attempts have been made in the past to improve physical properties of coatings by incorporating into a paint a chelate-forming compound such as a polyhydric phenol, e.g., pyrogallol, a phenol carboxylic acid, a chromium-containing complex salt, a phthalocyanine, a pyridine, a derivative thereof or the like and causing a chelating reaction between the coating and the surface of an iron substrate.
As an old instance of a paint of this type, there is known a solvent type resin formed by merely incorporating tannin in a linseed oil type resin or a drying oil (see the specification of British Pat. No. 826,564 and No. 826,566).
Later, R. N. Faulkner et al. developed a one-pack type solvent paint formed by introducing catechol, pyrogallol, gallic acid or gallic acid ester in the form of a covalent bond into a vegetable oil, a fatty acid ester, an alkyd resin, a vegetable oil-modified epoxy ester resin or a vegetable oil-modified polyamide resin by utilizing a catalyst such as a metal alkoxide. Paints of this type are disclosed in, for example, the specification of British Pat. No. 1,045,118 and U.S. Pat. No. 3,304,276 and U.S. Pat. No. 3,321,320, and Journal of the Oil and Colour Chemists' Association, 50, 524 (1967) published by the Oil and Colour Chemists' Association. Further, the specification of British Pat. No. 1,114,400 discloses a composition formed by reacting a styreneallyl alcohol copolymer with a gallic acid ester.
Chelate-forming compositions including an epoxy resin are also known in the art. For example, there are known chelate-forming resins formed by modifying a part of epoxy groups with a monobasic fatty acid and reacting the remaining epoxy groups with a fatty acid derivative having a chelate-forming capacity and containing in the molecule at least two adjacent phenolic hydroxyl groups and one free carboxyl group (see Japanese Patent Publication No. 2439/73); compositions comprising a chelate-forming epoxy-polyamide resin formed by reacting residual epoxy groups of a reaction product formed between an epoxy resin and a polyamide resin having an amine value of up to 10, with gallic acid and incorporating therein, a phosphoric acid derivative (Japanese Patent Publication No. 17443/73); and a one-pack type or two-pack type (an amine type curing agent being used) paint comprising a partially esterified product obtained by reacting a part of epoxy groups of an epoxy resin with salicylic or gallic acid or its ester, an epoxy, vinyl or fluoroethylene resin and an organic solvent as a diluent for the foregoing resin components (Japanese Patent Publication No. 4811/74 and Japanese Patent Application Laid-Open Specification No. 56226/73, No. 56228/74, No. 122538/74 and No. 122597/74).
In each of the foregoing known paints, a large quantity of an organic solvent is used, and hence, they are still insufficient with respect to safety and prevention of environmental pollution. Further, when ingredients of these paints are examined, it is seen that in each of these known paints, the epoxy resin which is the main ingredient, is used in the modified state. In other words, each of these known compositions is a one- or two-pack paint formed by reacting all or a part of epoxy groups in the epoxy resin as the main ingredient with a chelate-forming compound, and therefore, excellent properties inherent in the epoxy resin are drastically lowered.
Further, there are known solventless or high-solids epoxy resin coating compositions developed as coating compositions causing no environmental pollution or as labor-saving coating compositions. In coating compositions of this type, in view of efficiency of the coating operation, only epoxy resins with low viscosity can be used, and therefore, these compositions provide only coatings having poor corrosion resistance.
Some of the inventors previously proposed a completely solvent-free aqueous resin composition comprising (i) an aqueous dispersion of a resin containing in the molecule at least 2 epoxy groups and (ii) a reaction product formed between an amino type curing agent containing in the molecule at least 2 nitrogen atoms and active hydrogen atoms bonded thereto and protocatechuic acid and/or gallic acid is disclosed.
As a result of subsequent research work, it was found that aqueous coating compositions of this type involve various problems. More specifically, uncured coatings just after application of these compositions are easily dissolved or flowed by the contact with water, and freezing or destruction of the emulsion in the composition takes place at low temperatures. Therefore, the compositions are poor in storage stability at low temperatures. Further, when coating is applied at low temperatures, the coating is almost never cured uniformly and properties of the cured coating are extremely poor.